GSM-noise "buzz" issue

[Joerg Reisenweber]

See answers inline

Am Sa  20. September 2008 schrieb Werner Almesberger:
> Joerg Reisenweber wrote:
> > See placement of R4408. I think that's enough of a good story.
> 
> Hmm, I'm beginning to understand your MICBIAS theory. Both C3014
> and C3015 invitingly offer their MICBIAS end to any RF coming out
> of the R4401 and R4408 pair.
> 
> Luckily, the other capacitors in the area only show their GND end
> to this probable RF source.
> 
> No amount of component removal besides JK4401 could fix that, so
> that's consistent with experiment results.
> 
> A more specific test for MICBIAS contamination would be to remove
> R4305 and to bias the internal microphone from an external source
> (through a shielded cable ;-), e.g., across D4301, which is easily
> accessible. Has something like this been tried ?

YUP, long ago. Result: fix noise.


> 
> It's a pity that you don't have a proper RF generator. All those
> tests with the GSM module or an external phone as the RF source
> just introduce so many parameters completely outside your control
> that it's almost impossible to get meaningful results.

YUP. Also note that a proper signal-generator at +33~+39dB, without auxiliary 
stuff like faraday-room, very likely will lead to me unintentionally quitting 
my job, as soon as I start to operate it here in Germany. ;-) Maybe you even 
could visit me at Guantanamo then, which is more close to Argentina than 
Germany is. :D

 
> I wonder how hard it could be to build a little 1W RF generator
> that's tunable in the 1-2GHz range.

See above. 


> 
> Anyway, I disgress. What really confuses me is that we still don't 
> seem to have an explanation for why shorting MIC to GND doesn't
> seem to help. Or do we ? Or does it in the end ? :-)

GND actually is a part of the antenna-dipole. So maybe GND "is not good" 
there. Another explanation is around standing wave (which has voltage and 
current maxima), and impedance of source vs impedance of short-to-GND 
component + impedance of GND itself.
But I agree it's a little strange, at least we would expect different result 
from our gut-feeling and common experience on similar situations


> By the way, there are two goals here:
> 
> 1) Make sure we don't repeat the same mistake in the future. I
>    agree that you're perfectly right with just keeping RF out of
>    the can.
> 
> 2) Find a work-around for the GTA02 units already in the field.

My efforts are targeted on both, but I don't see the amount of progress and 
support I'd expect, on either of them.

For 1) I just won't stop to repeat to quote my own msg from April:
> Basic guidelines:
> =================
> If we go for a A7 PCB (please correct if in the following anything is
> missing or wrong):
> placement (layout) of *all* audio components has to be reconsidered!
> Separate shielding cage for audio section SHOULD be implemented.
> *All* copper rails *) creating a path from outside to inside of the audio
> cage MUST have EMI-filters *close* to the point they cross the cage border
> (usually those EMI-filters form 'gates' from outside to inside, by being
> mounted exactly ON the cage border line in special breakouts of the cage
> metal).
> There MUST NOT be any nonlinear component (=semiconductor) of audio
> circuitry outside the cage. This does not apply for specially EMI-hardened
> tested components like microphone, and for high-Z OVP-components.
> There SHOULD NOT be any component of audio circuitry outside the cage,
> except OVP.
> All audio copper rails SHOULD be inner layer of PCB, shielded on the outer
> layers by gapfree GND (or VDD) rails, and MUST NOT run in parallel to any
> high-current rail.
> All rails outside cage SHOULD be differential, high level (microphone!),
> low impedance.
> Differential lines MUST run in parallel on same layer.
> Layout of GND and supply-planes SHALL follow design guides of Wolfson (URL:
> tbd)
> All connectors to outside of housing SHOULD have EMI-filters [[edit: as
> close as possible to component]] on all lines
> except GND, because external cables are feeding remarkably high levels of
> RF to the PCB.
>
> *) that's one of the points why it's more reasonable to have a separate
> cage. We don't want the need for EMI-block on lines that aren't related to
> audio, but run to same cage. Also other (digital) circuitry may create
> noise *inside* the cage.

cheers
jOERG
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